1998 Jaguar XJ8 Oil in air filter

Recently purchase a xj8, the engine runs very well without any knocking noises. But I recently check the air filter, I find out that half of the air filter was soaked with some kind of black liquid, looks like oil. After removing the whole air filter, I saw the oil is coming from the air induction pipe. I really have no idea what is going on, can any people suggest what problem may it be, and around how mcuh would it cost to get it fix? Thanks a lot